1. Field of the Invention
The present invention relates to an X-ray CT system, a recording medium that stores a control program for the same, and a method of determining an imaging result in case of capturing CT images.
The present invention particularly relates to an X-ray CT system and A recording medium that stores a control program for the same that set CT-image imaging conditions based on a scout image of a subject.
Herein, examples of the CT-image imaging conditions include tube voltage, tube current, X-ray dosage, field of view (FOV), collimator aperture, slice thickness, wedge filter type, operation of the wedge filter, and position of the top board.
2. Description of the Related Art
In a conventional X-ray CT system, an engineer (operator) selects CT-image imaging conditions recommended by the system based on an obtained scout image. Additionally, the engineer visually confirms the size of the subject and sets the CT-image imaging conditions based on past experience, etc.
Additionally, a conventional X-ray CT system images a scout image of the subject and confirms the size of the subject based on the X-ray dosage introduced to detection elements during imaging of the scout image to set the CT-image imaging conditions and then images CT images under those CT-image imaging conditions. Alternatively, the scout images are each imaged by rotating and stopping the X-ray tube at each position located above the subject (in the direction of 0°) and located to the side of the subject (in the direction of 90°) and irradiating X-rays to the subject from these two directions. The engineer confirms the size of the subject based on each scout image imaged from the two directions to set the CT-image imaging conditions and then images CT images under those CT-image imaging conditions.
As described above, in a conventional X-ray CT system, CT-image imaging conditions are set based on either visual confirmation by the engineer or the scout image and CT images are then imaged under those CT-image imaging conditions.
Next, the following technologies have been adopted in X-ray CT systems. A technology has been adopted in which the engineer sets the tube current high when imaging bony regions such as the shoulder and pelvis in order to increase the X-ray dosage and sets the tube current low when imaging regions such as the chest (lungs) in order to decrease the X-ray dosage.
Recent X-ray CT systems adopt a technology that changes the conditions of the tube current depending on the region to be imaged or a technology that differentiates the X-ray dosage of the X-ray tube from the direction of 0° and 180° from the X-ray dosage of the X-ray tube from the direction of 90° and 270°. The engineer selects the above technologies according to the application.
Additionally, a plurality of detection elements is arranged in the direction of the fan angle of the X-ray tube. A wedge filter is provided between the X-ray tube and the subject in order to adjust the X-ray dosage of the X-rays that are introduced to each detection element by passing through or without passing through the subject. Therefore, for the central detection elements located on the centerline that bisects the fan angle, by assuming that X-rays that have been transmitted through the thick central part of the subject will be introduced, a large amount of X-rays is irradiated from the X-ray tube, whereas for the detection elements between the center and the ends, assuming that X-rays that have been transmitted through a thin part of the subject or that have not been transmitted through the subject will be introduced, a small amount of X-rays is irradiated from the X-ray tube according to the thickness of the subject that is penetrated.
When the X-rays are irradiated to the subject, the center of which has been located at the centerline that bisects the fan angle, according to the X-ray dosages described above, a large amount of X-rays is transmitted to the center of the subject and thereby sufficiently attenuated in the subject, and the X-ray dosage introduced to the center of the detection elements is therefore decreased. The introduction of data falls within the assumed count. A small amount of X-rays is either slightly transmitted through the subject or is introduced directly to the end of the detection elements without being transmitted through the subject.
Additionally, there is a technology that forms a preferred irradiation range for the diagnosis-target section by arranging a plurality of wedge filters with curves, the curvatures of which are each different, and moving the wedge filters in the body-axis direction of the subject (e.g., Japanese Unexamined Patent Application Publication No. 2007-267783).
However, in the abovementioned conventional X-ray CT system, in cases of setting CT-image imaging conditions in which the X-ray dosage is large relative to the size of the subject, the X-ray dosage introduced to the detection elements is excessively increased, causing an overflow of the detection elements and thereby generating artifacts (abnormal images). Unnecessary and excessive exposure to X-rays also leads to problems.
When the center of the subject is located by shifting the center from the centerline that bisects the fan angle, a large amount of X-rays is either slightly transmitted through the subject or is introduced directly to the detection elements in the center without being transmitted through the subject. The X-rays that are slightly transmitted and introduced to the detection elements in the center are indicated with a broken line in FIG. 7. The X-rays indicated with a dashed line undergo little attenuation in the subject and therefore, an X-ray dosage that is higher than expected is introduced to the detection elements in the center, leading to a problem that an overflow of the detection elements may be generated. When X-rays are introduced at a dosage that is higher than expected, ring artifacts (abnormal images) may be generated.